The nature of gas turbines is such that they emit small amounts of undesirable pollutants into the surrounding atmosphere. Although smoke, excess carbon monoxide and unburned hydrocarbons all constitute undesirable pollutants in the exhaust of gas turbines, it is the emissions of excess amounts of nitrogen oxides (NOx) which causes particular concern, as a result of the adverse effects attributed to these gases.
It is well known that lowering tile temperature of combustion will decrease tile concentration of nitrogen oxides in the turbine exhaust gases. It has also been demonstrated that burning the fuel with excess air, i.e., using a fuel-lean mixture in the combustion process, will accomplish a temperature reduction. However, the leanness of the fuel-air mixture required to effect a flame temperature reduction at full turbine load will not support a satisfactory flame under low load or under start-up conditions. When the latter conditions prevail, the turbine will operate at poor combustion efficiency, or not at all, if the same fuel mixture is used as at full load. Incomplete burning of the fuel mixture will occur, resulting in the presence of excessive amounts of carbon monoxide and unburned hydrocarbons in the turbine exhaust.
Current techniques for obtaining low levels of exhaust pollutants in a gas turbine include:
water injection into the combustor at additional customer operation cost--see, for example, U.S. Pat. Nos. 4,337,618; 4,290,558; 4,259,837; and 4,160,362; PA1 exhaust stack cleaning at additional installation and operation cost; and PA1 dry low NOx technology (fuel staging in the combustor) in a narrow, stable operating range--see, for example commonly assigned U.S. Pat. Nos. 4,292,801 and 4,982,570.
Existing combustion control systems which attempt to solve the problem of NOx emission by the use of variable geometry, whereby the fuel is burned with excess air, frequently operate with a relatively large variation in the pressure drop across the combustor. This variation occurs as the load on the turbine changes and it has an adverse effect on the overall operation. Further, because the control mechanism employed is usually integral with the combustor, the overall structure is mechanically complex and thus costly to build and maintain. As will be appreciated, combustors operate in a harsh external environment, up to 1000.degree. F. and 250 p.s.i.a. pressure. Moreover, the combustion system is surrounded by a pressure vessel which is expensive and difficult to remove for maintenance purposes. The reliability of sensors and mechanical devices in this environment has also proven problematic. Another disadvantage resides in the fact that existing combustors cannot be easily retro-fitted to incorporate this type of control system.
The levels of allowable exhaust pollutants for power generation equipment will be continually reduced on a worldwide basis. However, as noted above, lean, premixed combustion technology results in combustors which are stable only in a very narrow operating range. Operating in this narrow range is difficult because the gas turbines used for power generation are required to deliver power over a wide range and not a single set point.
The operating range of the premixed combustors can be extended through the use of air staging. Air staging in a general sense may be defined as altering the distribution of air entering the combustor in a controlled manner during operation. In commonly assigned U.S. Pat. No. 4,255,927, a combustion system for gas turbines is disclosed in which air flow from the compressor is directed to both the reaction zone and to the downstream dilution zone in a manner which permits variable inverse proportioning of the air supplied to these zones. There are significant hardware requirements in this system, however, for distributing the flow of air between the zones.
In commonly assigned U.S. Pat. No. 4,944,149, an air staging apparatus is disclosed wherein a ring provided with a plurality of holes is adjustable axially relative to the combustion liner to selectively cover and uncover dilution air holes in the liner to thereby adjust the amount of air flowing into the dilution zone. Since the movable hardware is located within the liner, maintenance is problematic.
It is the principal object of this invention to provide a simplified but nevertheless novel apparatus for providing air staging which will not decrease plant reliability or increase maintenance costs.
More specifically, the object of this invention is to overcome the drawbacks of the prior art without hindering maintenance and reliability caused by mechanisms within the pressure vessel to control air staging. By adding external air staging to existing dry low NOx combustors, it is possible to increase the stable operating range of the combustor. This increase in range is desirable because gas turbines used for power generation must operate over a wide range of conditions, and not merely at a single set point.
In current design gas turbine approximately 90% of the total compressor discharge air must flow 10% through the combustor at all times (the remaining of air is leakage or used for cooling the hot gas path). The external air staging apparatus of this invention will allow the distribution of air into the combustor such that a large percentage of the air (approximately 20%) can bypass the combustion reaction zone by entering the combustor via dilution holes in the dilution zone.
Thus, in accordance with one exemplary embodiment of the invention, air staging is provided which requires no internal mechanical devices or sensors. The system is designed to locate these components externally of the pressure vessel where high reliability is demonstrated and maintenance easily achieved. In accordance with the invention, compressor discharge air for air staging is removed from the pressure vessel at (preferably) between 4 and 6 locations about the vessel. The air flows via a corresponding number of pipes to air dilution control valves which control amounts of compressor discharge air subsequently introduced to a common manifold which distributes air to each combustor in the pressure vessel. To this end, each combustor is provided with an annular manifold for feeding air into the combustion chamber through a plurality of dilution air feed tubes.
In its broader aspects, therefore, the present invention provides, in a gas turbine combustion system which includes a plurality of combustors within a pressure vessel, each combustor including a combustion liner defining a combustion chamber having a reaction zone and a dilution zone, the liner in the dilution zone provided with a plurality of circumferentially spaced dilution air feed holes; a flow shield surrounding each combustion liner in radially spaced relation thereto for feeding compressor discharge air to the combustion chamber, an improvement comprising air staging apparatus for controlling the amount of compressor discharge air introduced into each combustion chamber dilution zone via the dilution air feed holes comprising a plurality of pressure vessel extraction ports downstream of the combustion liner for introducing compressor discharge air into a first manifold externally surrounding the pressure vessel; and a single manifold feed pipe extending between the first manifold and each the combustion liner.
Since, essentially, all of the hardware associated with the air staging control system is located externally of the pressure vessel, maintenance and/or replacement of parts is facilitated with an overall reduction in cost.